1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the current invention relates to a new sandwich electrode design and a process for manufacturing the same. Sandwich electrodes are useful as the cathode in primary lithium cells and as the positive electrode in secondary lithium ion cells. These designs make such cells particularly useful for powering implantable medical devices.
2. Prior Art
Early medical devices in many cases used at least two lithium electrochemical cells in series as their power source. However, the electronic circuits in these devices now consume less energy than before. This makes it currently possible to use a single lithium cell as a reliable power source. With a unitary cell design, the requirement for high power density in many applications is even greater as the result of lowered pulsing voltage. Thus, a large electrode surface area is needed to accomplish this requirement. However, as the electrode surface area increases, more inert materials (current collector, separator, etc.) are introduced into the system. As a result, the cell""s volumetric capacity is decreased. Another concern is medical device longevity, which is dependent on the cell""s capacity and power efficiency.
An attempt to use high capacity materials, such as CFx, by mixing it with a high rate cathode material, such as SVO, is reported in U.S. Pat. No. 5,180,642 to Weiss et. al. However, electrochemical cells made with these cathode composites have relatively lower rate capability. The benefit of increasing the cell theoretical capacity by using CFx as part of the cathode mix is balanced, in part, by lowering its power capability in a high rate discharge application, such as is encountered in an implantable cardiac defibrillator.
A significant solution to this problem is described in U.S. patent application Ser. No. 6,551,747 to Gan entitled Sandwich Cathode Design For Alkali Metal Electrochemical Cell With High Rate Capability by Gan et al., which is assigned to the assignee of the current invention and is incorporated herein by reference. This application describes a new sandwich electrode design using silver vanadium oxide (SVO) and a fluorinated carbon (CFx). An exemplary sandwich electrode has the following configuration:
SVO/current collector screen/CFx/current collector screen/SVO.
However, if the openings in the current collector screen are too large, there can be communication of one of the active materials to the other side of the current collector during the manufacturing process. This xe2x80x9ccontaminationxe2x80x9d is undesirable as it detracts from discharge performance. Specifically, SVO is of a higher rate capability, but a lower energy density than CFx. Therefore, contamination of the interface between the current collector and one of the active materials by the other is undesirable as it defeats the purpose of having the respective active materials segregated on opposite sides of the current collector in the first place.
To maintain the improved discharge capability of a cell containing a sandwich electrode, it is necessary to maintain direct contact of both the first and second electrode materials with the opposed sides of the current collector. A good contact or adhesion translates into good interfacial conductivity during discharge. Although it is clear in theory, in practice this interfacial conductivity is highly influenced by the manufacturing methods or processes. When the current collector is a screen, it is possible for some of one of the electrode materials to pass through the current collector openings and become trapped between the other electrode material and the current collector. This leads to decreased interfacial conductivity between the current collector and the xe2x80x9ccontaminatedxe2x80x9d first electrode material.
Thus, the present process consists of having one of the electrode active materials in a cohesive form incapable of moving through the current collector to the other side thereof. However, in an un-cohesive form, the one electrode active material is capable of communication through the current collector. The other or second active material is in a form in-capable of communication through the current collector, whether it is in a powder form, or not. Then, the assembly of first active material/current collector/second active material is pressed from either the direction of the first electrode active material to the second electrode active material, or visa versa.
In that respect, the present invention is directed to an electrochemical cell, comprising: an anode; a cathode, wherein at least one of the anode and the cathode is characterized as having been formed by a method consisting essentially of: positioning a first electrode active material into a pressing fixture; positioning a first current collector screen on top of the first electrode active material; positioning a second electrode active material on top of the first current collector screen; positioning a second current collector screen on top of the second electrode active material; positioning a third electrode active material on top of the second current collection screen, thereby forming an electrode assembly; and pressing the electrode assembly to form the electrode; and a separator electrically insulating the anode from the cathode; and an electrolyte activating the anode and the cathode, wherein when the first and third electrode active materials are in an un-cohesive state, they are of an un-cohesive size less than an opening size of at least one opening of the current collector screen and capable of moving through the at least one opening, and wherein the first and third electrode active materials are in a cohesive form incapable of moving through the at least one opening in the current collector screen and wherein the second electrode active material is in a form incapable of moving through the at least one opening in the current collector screen, and the electrode assembly is characterized as having been pressed from the direction of either the first electrode active material to the third electrode active material or from the direction of the third electrode active material to the first electrode active material.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.